This invention relates to pre-plasticizing injection molding machines using shooting pots as intermediate reservoirs of melted plastic resin. More particularly this invention relates to apparatus and methods for improving colour changing of resins to reduce changeover time and resin wastage.
A conventional two stage or xe2x80x9cpre-plasticizingxe2x80x9d injection molding machine comprises, in general, a plasticizing component having a screw in a heated barrel and an injection component having an injection plunger. Polymer resin plasticized in the plasticizing component is transferred to the injection component""s chamber, or xe2x80x9cshooting potxe2x80x9d, by way of a feed channel and is injected into a mold cavity through a nozzle by the injection plunger. Conventionally, a distributor or ball check valve in the feed channel prevents resin in the shooting pot from flowing backwards to the plasticizing component when the injector plunger injects the resin.
When the resin is changed to a different colour or different resin type, the previously processed resin must be purged from the extruder and the shooting pot before processing using the new colour or resin can begin. Typically, the new resin is used to purge the old resin.
U.S. Pat. No. 4,290,701 to Schad shows a typical pre-plasticizing injection unit for a molding machine. The shooting pot design shown in the ""701 patent is a xe2x80x9clast-in-first-outxe2x80x9d style because the same port to an injection cylinder is used to both fill the cylinder and empty it. Although the injection plunger has a profile matching both the cylinder bore and an internal surface of a shooting pot head covering an end of the cylinder, and the injection plunger can be xe2x80x9cbottomed outxe2x80x9d when advanced until it contacts the cylinder head, some resin remains behind when purging. Because the xe2x80x9cremainingxe2x80x9d resin may remain in the shooting pot longer than most of the through-feeding resin, it can degrade and mix into the melt stream causing imperfections in the molded articles. Multiple feedings of the new resin to and from the shooting pot are typically required to clean out the prior resin. This can be both time consuming and wasteful of expensive resin material. For example, if a large component such as a garbage bin is being molded, changeover may require 50 to 100 cycles and takes a significant amount of time to complete.
U.S. Pat. No. 2,950,501 to Harkenrider is an early example of a xe2x80x9cfirst-in-first-outxe2x80x9d shooting pot design. By placing an entry port to the shooting pot at the maximum volume location of the plunger, the first resin entering the cylinder will move to the cylinder head end of the injection cylinder adjacent an exit port so that when the plunger is advanced, the xe2x80x9cfirst-inxe2x80x9d resin is injected into the mold, or purged as the case may be. Similarly, the xe2x80x9clast-inxe2x80x9d resin will be the last to be expelled. The disadvantage of this design is that variable shooting pot volumes, requiring a different shooting pot piston starting position other than maximum volume cannot be easily accommodated. The piston must be fully retracted each time the shooting pot is to be loaded prior to a molding cycle. Partial fillings cannot be made without risking air entering the cylinder and mixing with the resin.
U.S. Pat. No. 5,380,186 to Hettinga et al. shows a shooting pot cylinder having a hollow piston through which the resin is fed to the shooting pot. This xe2x80x9cfirst-in-first-outxe2x80x9d style allows for various volume fillings of the cylinder, but means that the construction of the injection unit becomes very long as the extruder and shooting pot are mounted on the same center line.
U.S. Pat. No. 5,814,358 to Bock shows another arrangement of a xe2x80x9cfirst-in-first-outxe2x80x9d shooting pot and extruder. In this arrangement, the shooting pot plunger is mounted inside a moveable shooting pot cylinder which is itself mounted inside an outer heated cylinder. The complication of this construction adds more places for the resin to xe2x80x9changupxe2x80x9d and degrade and increases cost.
U.S. Pat. No. 5,858,420 to Szajak et al. shows a xe2x80x9cfirst-in-last-outxe2x80x9d arrangement having separate channels used for the transfer of resin and the injection of resin. Transferred resin is brought into the shooting pot head behind resin retained from previous shots by the use of a plurality of channels communicating with the periphery of a conical tip of the shooting pot plunger head. The melt is therefore distributed more uniformly into the shooting pot and during injection, resin retained from the last cycle is injected first through the separate injection channel. Although this arrangement gives improved purging, some stagnation nevertheless exists requiring numerous cycles to properly effect a resin type or colour changeover.
A method is provided for purging a shooting pot having an injection plunger slidably received in an injection cylinder. The method comprises the steps of:
i) moving the injection plunger to a purging position;
ii) arresting the injection plunger in the purging position;
iii) injecting melt into the injection cylinder through a melt inlet; and,
iv) discharging melt through an outlet remote from the inlet.
Preferably, the inlet and outlet are positioned to cause injected melt to sweep substantially an entire volume of the injection cylinder ahead of the injection plunger as injected melt flows between the inlet and the outlet.
In one embodiment, in the purging position, a tip of the injection plunger is positioned adjacent a corresponding shooting pot head covering an end of the injection cylinder. The melt inlet is adjacent an outer periphery of the tip and the melt outlet is generally co-axial with the injection cylinder.
In an alternate embodiment, the injection cylinder is provided with at least one outlet ordinarily covered by the piston and in moving the injection plunger into the purging position, the injection plunger is overstroked to uncover the outlet.
The outlet in the alternate embodiment may be a drool port which leads from a channel extending circumferentially about the injection cylinder and in the purging position the injection plunger at least partially uncovers the channel.
The position of the injection plunger in the purging position may be incrementally variable to vary a degree to which the circumferential outlet is uncovered to optimize purging.
A shooting pot is provided for an injection molding machine. The shooting pot has an injection cylinder with a shooting pot head covering an end thereof. An injection plunger is slidably received in the bore in a substantially fluid sealed manner. A melt passage extends through the shooting pot head into a first end of the bore. A drool port is located distal the melt passage for discharging melt seeping between the injection plunger and the injection cylinder. An actuator moves the injection plunger reciprocally along the bore over a first distance corresponding to an injection stroke. The injection plunger covers the drool port over the injection stroke. An overstroker is provided to move the injection plunger along the bore to a purging position in which the drool port is at least partially uncovered. The shooting pot has an arrestor for arresting the injection plunger in the purging position.
The actuator may also act as the overstroker and a channel may extend circumferentially about the bore at the drool port. The shooting pot may also include a distributor valve for selectively providing fluid communication between the melt passage and either of an extruder feed line and a nozzle.
An alternate embodiment shooting pot is provided which has an injection cylinder with a bore, a shooting pot head over an end of the bore and an injection plunger slidably received in the bore in a substantially fluid sealed manner. At least one inlet extends through the shooting pot head and at least one outlet also extends through the shooting pot head. The shooting pot includes a reconfigurable flow controller moveable between load, eject and purge configurations. In the load configuration, the flow controller allows melt flow into the inlet and prevents melt flow at the outlet. In the eject configuration, the flow controller prevents melt flow at the inlet and allows melt flow out of the outlet. In the purging configuration, the flow controller allows melt flow into the bore through the inlet and out of the bore through the outlet.